The present invention relates to a method for converting an analogue signal representing a runlength limited binary signal with runs having a length of at least N into a binary output signal.
n a d=1 RLL code the shortest allowable runlength is an I2 (two equal 2Nxe2x88x921consecutive binary symbols), translating to a (highest) frequency component of 1/4T, where 1/T is the channel bit rate. The physical length of a channel bit on the disc is chosen in such a way as to ensure that the highest frequency allowed by the code is still lower than the cut-off frequency of the optical channel (determined from the parameters of the optical pick-up). This provision ensures that the smallest marks (pits or lands) will be resolved during disc read-out. However, in order to achieve the aimed capacity of around 25 GB in DVR, the recording density is increased. This translates to smaller marks on the disc, the resolution of which becomes increasingly difficult. The same phenomenon is observed when the writing speed is increased; this leads to transition shifts (writing jitter) in the opposite direction for rising and falling edges of small runlengths on the disc. This again results in the shortening of small runlengths.
A direct consequence of density and/or writing speed increases is the fact that traditional techniques for data detection, such as threshold detection, fail to achieve adequate performance. Even more sophisticated detection methods, such as Runlength Pushback detection as described in EP 948844 A1, suffer significant performance losses at the high capacities aimed for DVR. The obvious solution to the problem is to resort to Maximum Likelihood Sequence Detection (MLSD) techniques, which are known to achieve optimum performance under certain conditions. MLSD techniques indeed seem to do the job, and exhibit significant robustness with respect to density increases. However, their enhanced performance comes at the price of bit-recursive operation, a fact which may prohibit their implementation at the very high data rates that are likely to emerge.
It is an object of the invention to provide a method which has a performance comparable to that of MLSD methods, but which avoids bit-recursive operation. In order to achieve this object the method according to the invention for converting an analogue signal representing a runlength limited binary signal with runs having a length of at least N into a binary output signal comprises the following steps:
a. sampling the analogue signal,
b. generating an intermediary binary signal on the basis of the sampled analogue signal,
c. identifying binary sequences within the intermediary binary signal having a length greater than 2Nxe2x88x921, the sequences comprising a set of successive samples having the same binary value, and which set borders on samples having an opposite binary value and for each such binary sequence,
d. selecting an analogue sequence in the sampled analogue input signal which corresponds to the identified binary sequence,
e. searching for the existence of a plurality of significant local extrema within the selected analogue sequence,
f. if such a plurality of significant local extrema is found partitioning the identified binary sequence into a corresponding plurality of smaller runs, comprising at least one run of length N.
The method according to the invention avoids recursive operations, but detects the binary signal with a relatively high accuracy. The latter is based on the following observations.
The proposed detection scheme is tailored to high capacity and/or high speed optical recording systems using a d=1 channel code, such as DVR RW. A study of the bit-errors that are usually encountered in such systems was first conducted. An analysis of the bit-errors left after application of (a modified version of) the run-detector of EP 948 844, that corrects only for violations of the d=1 constraint, revealed a single most important category of errors in the detected bit-stream. These errors occur by miss-detection of I2""s in the HF signal. Both samples of these I2-runs are either not high (for an I2-land) or low (for an I2 pit) enough to cross the threshold level of the detector, and are therefore detected as parts of their surrounding runs (of opposite polarity). The result is that the two pits (lands) surrounding the I2 land (pit) are joined by the miss-detected I2, to form a longer combined run.
The problem of miss-detected I2s becomes more severe as recording density and/or recording speed is increased, and accounts for an increasing percentage of errors. An important observation from the performed experiments is that if all erroneous I2""s can be detected and corrected, then near-MLSD performance can be achieved. It is therefore adequate, at least in terms of bit-errorrate performance, to concentrate only on this type of errors.
The proposed detection methodology is in particular suitable for the correction of a certain category of errors, which occur in high-density and/or high writing speed d=1 recording. Recently, other sub-optimal detectors have been introduced for ROM applications.
These detectors are in turn focused on the correction of errors occurring in ROM media, under DVD-like conditions. Since the sources of errors in ROM media are different from those encountered in the present case (d=1, DVR RW-like conditions), these detectors are not considered here.